The present invention relates generally to interferometers for precise measurement of microscopic distances or level differences or microscopic asperities of the surface of an object, which are suitable for use in precisely detecting defects in the surface of a disk-shaped recording medium such as a magnetic or optical disk. More particularly, the present invention relates to an improved interference detecting system for use in an interferometer to detect an interference phase or interference intensity.
In recent years, most personal computers come with a hard disk device as their standard equipment, and a great majority of these hard disk devices have a capacity as great as a few giga-bytes. Further, there has been an increasing demand that notebook-sized personal computers be capable of incorporating therein an internal hard disk device that achieves high-density recording and is yet compact in size.
Generally speaking, in order to achieve increased recording density by the hard disk device, it is necessary to minimize the floating amount or distance of its magnetic head from the surface of the magnetic disk, say, down to the order of 20 to 50 nm. When the magnetic disk for use with such a hard disk device is inspected or tested for surface defects, the inspection has to be performed with such detection accuracy corresponding to the floating amount of the magnetic head.
So far, the inspection for magnetic disk surface defects has been carried out using a device which is commonly called a "glide tester". Each of the traditional glide testers causes the magnetic disk to rotate with a preset floating amount, during which it detects how many times the magnetic head has collided with abnormal or intolerably-high projections on the disk surface. Then, on the basis of the detected number of the collisions, the glide tester determines the glide levels (i.e., heights of the projections on the surface) of the magnetic disk.
However, if the magnetic head is set to an extremely small floating amount, e.g., on the order of 20-50 nm, then the number of the collisions becomes correspondingly greater. The increased collisions of the magnetic head against the disk surface projections are quite undesirable in that they would often damage the magnetic head and thus require replacement of the damaged head, positioning of a new (replacing) magnetic head, etc., which are very time-consuming.
Japanese Patent Laid-open Publication No. HEI-8-114431 discloses one example of the glider tester, which is designed to detect abnormal projections on the disk surface by measuring the floating amount of the magnetic head in an optical manner. However, the disclosed test glider thus arranged still can not provide a solution to the above-discussed problem.
To solve the problem, the recent trend in the art is toward optically detecting heights of abnormal projections on the disk surface in order to perform a disk surface inspection similar to that performed by the glide testers, and devices intended for such an inspection are called "optical glide testers". One example of such optical glide testers is disclosed in Japanese Patent Laid-open Publication No. HEI-8-220003, which is designed to determine surface defects, in the form of asperities, of the disk by irradiating light onto the disk surface at a predetermined angle and closely examining the reflections of the light off the disk surface. The disclosed optical glide tester requires very complicated know-how for the examination of the reflected light and analysis of the surface asperities based thereon, but, despite the use of the complicated know-how, it could not perform the surface defect inspection with ease.
Among the conventionally-known optical measuring techniques is a precise interferometer-based measuring technique. Generally, the interferometer divides coherent laser light into two light beams via a beam splitter so as to irradiate one of the divided beams (reference beam) onto a predetermined reference surface and irradiate the other divided beam (measuring beam) onto a predetermined surface to be measured (hereinafter called a "test surface"). Then, the interferometer combines together respective reflections of the two irradiated beams and detects light interference conditions in the combined reflection (interference light), to thereby detect level differences in stepped regions and asperities of the disk surface. In this case, the interference detection is generally made on the basis of two detecting principles: an interference-phase detecting principle; and an interference-intensity detecting principle. The interference-phase detecting principle is intended to detect a phase of the interference light, while the interference-intensity detecting principle is intended to detect intensity of the interference light. If the reflections of the reference and measuring beams are exactly in phase with each other, then the interference light presents no phase difference relative to a predetermined reference phase (e.g., a phase of the reference beam) and presents maximum intensity. However, once the reflection of the measuring beam gets out of phase with the reference beam, there would occur a corresponding phase difference in the interference light along with corresponding attenuation of the interference light intensity. Consequently, any level differences in stepped regions and asperities of the test surface can be detected irrespective of which of the interference-phase and interference-intensity detecting principles is employed for the surface defect inspection.
Thus, a more sophisticated or advanced optical glide tester may be provided by applying the known detecting principles for an interferometer; however, mere application of the known detecting principles would result in the following problem. Namely, the interference-phase and interference-intensity detecting techniques employed in the conventional interferometers are directed only to light interference detection based on "distance components", such as level differences in stepped regions and asperities on the test surface, and never take into account variation in reflectivity of the test surface. Consequently, the significant problem would be encountered that the detecting accuracy decreases due to successive variation in the reflectivity of the test surface. More specifically, due to the fact that variation in the reflectivity of the test surface causes variation in the interference light level involving fluctuations in level at and around its amplitude center (zero phase), it is difficult for the interference-phase detecting technique to accurately detect the the zero-cross point phase for precise measurement of phase differences, which would thus unavoidably lead to reduced detection accuracy. Further, because the variation in the reflectivity of the test surface involves fluctuations in the interference light level, the interference-intensity detecting technique can not detect interference intensity that exactly corresponds only to "distance components" such as level differences in stepped regions and asperities of the test surface.